This invention relates generally to a process for making thin-wall containers from sheets of thermoplastic resin. More particularly, but not by way of limitation, this invention relates to a process for blow molding a thin-wall container from a sheet of thermoplastic resin.
Various processes and apparatus have been utilized in the past for forming containers and the like from thermoplastics. A number of these processes utilize a preform member that is placed into a mold and then heated and enlarged, usually by pressure until it reaches the configuration defined by the interior of the mold. Sometimes, the apparatus for expanding the preform member includes a rod or piston that is moved in an axial direction to insure proper elongation of the preform. Containers formed in this method, were, assuming the temperatures to be proper, biaxially oriented, that is, the molecules therein are aligned axially and radially whereby the finished product is substantially stronger than a container of the same configuration that is blow formed at a non-orientation temperature. Such apparatus and procedure are described in U.S. Pat. No. 3,900,120 issued to Thomas F. Sincock on Aug. 19, 1975.
U.S. Pat. No. 3,814,784, which issued June 4, 1974 to William D. Wolf, describes a plug assisted, blow molding procedure wherein a sheet of thermoplastic material is placed across the face of the mold and a low pressure applied to one side of the sheet causing a hemispherical billowing of the sheet. The plug is then brought into engagement with the sheet, forcing the sheet into the product mold. After this occurs, pressure is applied within the mold to cause the plastic to form the final configuration of the container. The expansion of the sheet initially is at a temperature where the resin is in a plasticized state and is accomplished by applying pressure from above. The plastic is then cooled and the plug brought into engagement with the plastic. The temperature of the resin when being forced upwardly by the plug and when being expanded into the final product configuration will be in the orientation temperature range of the particular resin. Thus, the process taught by Wolf produces a product that is a biaxially oriented deep container of relatively thin-walled configuration.
An additional Wolf patent, U.S. Pat. No. 3,737,494, issued June 5, 1973 describes a very similar process and apparatus for producing thin-walled products. It is to be noted that the apparatus taught by Wolf and the process used in his patents are of essentially open mold configuration. When the plunger is forced all the way into the product mold, the periphery of the mold carrying the plunger is brought into sealing engagement with the clamping ring of the upper mold portion to close the mold.
Manifestly, it is highly desirable to form the containers from a sheet since it is possible to form a sheet with multiple layers of materials that include those materials providing the product with high thermal qualities, low temperature qualities, and to provide gas barriers to prevent either the ingress of oxygen into the product contained within the final container or egress from such container of gases such as CO.sub.2 from carbonated drinks. It is also highly desirable that the containers be produced in a closed mold configuration since the temperatures thereof can be more carefully controlled and the pressures necessary to the formation and orientation of the product can be more easily attained.
Accordingly, it is the object of this invention to provide an improved apparatus for thermoforming an article from a thermoplastic resin sheet wherein the product is of relatively thin wall, biaxially oriented for strength and formed in a closed mold wherein the desired temperature and pressures can be attained and controlled.